1. Field of the Invention
The present invention relates to the user scheduling technology of the multiple input multiple output (MIMO) system, especially the MIMO communication system based on random beamforming and its user scheduling method.
2. Description of the Related Art
The future wireless communication system is required to support the extremely high speed data traffic, such as the videoconference, the video-on-demand and the interactive video game, etc. It should support up to 100 Mbps for high mobility traffic and up to 1 Gbps for low mobility or fixed wireless traffic.
The data rate of one wireless channel is equal to the product of its spectrum width and the spectrum efficiency of the adopted technology. In order to improve the data rate, the spectrum width of the channel or the spectrum efficiency of the adopted technology should be improved. However, since the frequency resource is limited, the communication speed cannot be raised by infinitely increasing the spectrum width. Improving the spectrum efficiency of the adopted technology is an optimal solution.
There are generally two methods to improve the spectrum efficiency. One is by the physical layer technologies such as advanced coding technologies and signal processing technologies etc. to improve the link level spectrum efficiency and the other is by high level control to realize more flexible resource allocation to improve the system level spectrum efficiency. MIMO technology and Channel-Aware User Scheduling are the corresponding two methods to realize the aims.
The so-called MIMO technology means that multiple antennas are mounted at both the transmitter and the receiver in a communication system. The MIMO technology also includes that multiple antennas are mounted at either side, i.e., the single input-multiple output (SIMO) and the multiple input-single output (MISO). Different antennas are physically separated, and are generally regarded as introducing an additional signal domain-a space domain, into the communication system.
Although the spatial resource can provide much performance gain, it is difficult to be utilized because of the uncertainty for changing with the object physical condition. A better control technology is essential to better utilize the spatial resource.
Next the user scheduling technology will be discussed. Currently two basic user scheduling technologies are adopted in the wireless communication system. One is Round Robin scheduling, which refers to a process in which channels are allocated to all the users circularly. This method guarantees the delay characteristic and the equitableness between users as a traditional switch does but the performance is not improved. The other is maximum carrier/intertrace (MaxC/I) scheduling (Channel-Aware User Scheduling), which dynamically allocates the right of accessing the channel to the user with the MaxC/I (simply represented as max|hk|) according to the channel fading condition hk (in single antenna system, it is a complex number scalar quantity) of the user. The performance of the system can be improved greatly and the performance gain through MaxC/I scheduling is called multiuser diversity.
However, since channel user scheduling allocates common channels according to the channel condition, it depends more on the channel condition. The system performance will be greatly reduced in some specific channel conditions.
FIGS. 1(a) and 1(b) show the system architecture diagram with one transmitting antenna at the base station (transmitter) and two users (receiver). In the system, channel user scheduling allocates common channels according to the channel condition.
In FIG. 2, (a) show the channel gain when the channel condition is good; (b) shows the channel gain when there is the line of sight (LoS) in the channel; (c) shows the channel gain when the system is in slow fading.
In FIG. 2, curve 1 shows the channel gain curve of user 1 changing with time, curve 2 shows the channel gain curve of user 2 changing with time and the dashed line shows the average channel gain curve of the system changing with time. FIG. 2(a) shows the system allocates common channels according to the channel gain of user 1 and user 2 at different times; the common channels are allocated to user 1 in interval 0−t1 and to user 2 in interval 0−t2, etc., which are represented respectively by “1” and “2” on time axis. The channel gain of the system is the upper envelopes of curves 1 and 2, and the dashed line stands for the average channel gain curve of the system.
Comparing (a) and (b) reveals when there is the LoS in the channel, since the LoS will reduce the fluctuation of channel coefficiency, the possible average channel gain of the system will be reduced. (b) and (c) show (intervals in the brackets) when the system fading is comparatively slow, the transmission delay will be comparatively larger.
To solve this problem, P. Viswanath, D. N. C. Tse and R. Laroia, etc. “Opportunistic beamforming using dumb Antennas”, IEEE Trans. Infor. Theory, Vol. 48, No. 6, pp. 1277-1294. June. 2002, proposed a solution.
In the method, suppose there are nT antennas at the base station and every user has one receiving antenna, then the channel of the user is a vector hkεCnT×1. Before transmitted, the data signal will be multiplied with a nT dimension random complex vector wεCnT×1, and the data signal will be transmitted from all the nT antennas. The channel gain detected by each user is the equivalent channel gain GNk=|h*kw| with reference to the actual channel and the transmit vector. Each user feeds back the detected equivalent channel gain to the base station, which allocates the channel to the user with the maximum equivalent channel gain.
For example, in FIGS. 1a and 1b, the user with the maximum equivalent channel gain is right inside the transmit beam generated by the transmit vector hk. Changing the random complex vector W can change the statistics characteristics (such as correlation and time-varied characteristics) of the equivalent channel gain to meet the request of user scheduling. However, with this method, only one user can be scheduled at one time. For example, user 1 is scheduled at t1 and user 2 is scheduled at t2. In this way, a lot of space resource is wasted and the throughput of the system is reduced in high SNR region.
In order to overcome this shortcoming, R. Laroia, J. Li, S. Rangan and M. Srinivasan, etc., “Enhanced opportunistic beamforming,” IEEE VTC2003-Fall, Vol. 3, PP. 1762-1766, October 2003, proposed multiple random beamforming.
Since nT transmitting antennas can support nT independent transmit beams theoretically, the method generates nT random transmit vectors wn (in which n=1, . . . nT) and every user will then feed back a best transmit beam and the corresponding equivalent channel gain. Maybe more than one user will select the same transmit beam but the base station will allocate the beam to the user with the maximum equivalent channel gain. The method has the following shortcomings:    1) The probable interference between random transmit beams is not taken into account. For example, in FIG. 3, since user 1 and user 2 correspond to the main lobes of beam 1 and beam 2 respectively, user 1 and user 2 are scheduled according to the method. However, when the side lobe of beam 1 interferes user 2 greatly, the actual performance of user 2 corresponding to beam 2 may be not as good as that of user 3. So interference should be taken into account in multiple user scheduling.    2) nT users have to be scheduled at one time. However, the channel condition is always in change in practice and for most of the time this amount of users cannot be supported simultaneously. Compulsive multi-user support will cause the decline of the system performance.
In addition, how to schedule the users with a plurality of antennas is not disclosed in the method.
J. Chung, C. S. Hwang, K. Kim, and Y. K. Kim, etc., “A random beamforming technique in MIMO systems exploiting multiuser diversity,” IEEE JSAC, Vol. 21, No. 5, June 2003, proposed a method using random beamforming when the user has a plurality of antennas. However, all the beams are allocated to the same user according to this method, which is apparently not the optimal.